The most common orthopedic condition for which professional medical treatment is sought is lower back pain. Although many factors may be responsible for causing lower back pain, a principal factor is damage or degeneration of an intervertebral spinal disc resulting in impingement on the nerve system, specifically the spinal cord, located within the spine. Such impingement may result in, for instance, loss of mobility, urinary and fecal incontinence, and sciatica or pain experienced in the extremities.
Damage to or degeneration of a spinal disc can result from a number of factors such as abuse or age. The disc itself is composed primarily of an annulus and a nucleus contained therein. The annulus is a fibrous annular piece that connects to the adjacent vertebrae and contains the nucleus, which is in turn a gel-like viscous material capable of shock absorption and flowable to permit poly-axial rotation and resilient compression of the vertebrae and spine. Most frequently, disc degeneration results from damage occurring to the annulus such that the flowable nucleus material may leak or seep out of the annulus. Disc degeneration also can occur in other ways, such as by being deprived of nutrient flow leading to a dried disc susceptible to damage. Because the nuclear material is flowable, extensive damage to the annulus is not necessary for leakage to occur.
Currently, approaches to treatment of spinal problems directly affecting the spinal cord are numerous. For instance, immobilization and high doses of corticosteroids may be employed. The dominant surgical procedures for treatment of these problems are spinal fusion and discectomy. Fusion is a method where adjacent vertebrae are immobilized so that they permanently secure to each other by having bone growth between and to the vertebrae, while discectomy involves removal of a portion or an entirety of a spinal disc.
However, the current practice of each of these procedures typically has certain limitations. With fusion, making a portion of the spine generally rigid produces a reduction in mobility, and drastically alters normal load distribution along the spinal column. Due to these factors, the non-fused portions of the spine experience stress and strain that are significantly increased over normal physiological motions. The increased stress and strain on the non-fused portions may lead to accelerated disc degeneration of the non-fused portions, particularly the adjacent levels of the spine.
Discectomy is effective for relieving sciatic pain by removing the damaged or herniated disc tissue compressing the spinal nerves. However, current discectomy often may lead to a reduction of the disc space between adjacent vertebrae, as well as instability in the affected portion of the spine. Such long-term effects with current discectomy often result in further surgery several years after the initial discectomy surgery.
In an alternative spinal surgery, a disc arthroplasty restores or reconstructs the disc using a prosthesis to replace a portion or entirety of the damaged disc. The primary objective of disc arthroplasty is to restore or maintain the normal disc anatomy and functions, while addressing and treating the causes of the pain. However, prosthetic disc implants have problems due to the complexity of the natural disc structure and biomechanical properties of a natural spinal disc. As used herein, the term natural refers to normal tissue including portions of the spine and the disc.
Two types of prostheses for disc arthroplasty are currently believed to merit further development by medical science and research. One type is a total disc prosthesis, or TDP, where the entire spinal disc is replaced after radical discectomy. A typical TDP includes structures that attempt to together mimic the properties of a natural disc.
The other type is a disc nucleus prosthesis, or DNP, that is used to replace only the nucleus of a spinal disc after a nucleotomy while retaining the annulus of the disc and, possibly, the endplates intact. As discussed above, failure of the natural disc does not require extensive damage to the annulus. An undamaged annulus, however, would often be capable of retaining a non-flowing prosthetic nucleus. Implantation of a DNP involves making a small incision in the annulus, clearing of the natural nucleus from the annulus through the procedure known as nucleotomy, and inserting the DNP through, and then within, the annulus. Accordingly, DNPs are typically smaller and require less extensive surgery than TDPs while still mimicking some of the biomechanical properties of a natural intervertebral disc.
Implantation of most DNPs with pre-formed dimensions requires a 5-6 mm, or larger, incision in the annulus for implantation and uses minimal disc tissue resection. Moreover, recovery and post-surgical pain are minimal due to the minimal invasiveness of the procedure, and interbody fusion remains a viable revision surgery. In addition, the incision in the annulus is kept as small as possible to minimize the potential for the implant to back out through the incision. The annulus itself is used to at least aid in maintaining the implant within the nuclear space. This permits the DNP to sit in the intervertebral space without anchors that violate the endplates of the vertebrae. As the annulus does not heal well and suturing the annulus is difficult due to its tissue properties, once the incision is too large, the ability of the annulus to retain the implant is diminished if not eliminated.
The risk of enlarging the incision on the annulus in the DNP procedure is increased because sizing tools are typically also placed through the incision. The size of the implant should match the size of the natural disc and/or nuclear cavity (i.e. the height of the space between adjacent vertebrae and the width and length dimensions or the footprint of the space within the annulus). If the implant is too large or too small, the implant may cause damage to the spine or pain to the patient. In order to determine the size of the nuclear cavity, conventional sizing tools such as a set of trial spacers disclosed by U.S. Pat. No. 6,478,801 are used. Each spacer has a different size and is sequentially inserted in the nuclear space in trial and error fashion until the trial spacer fits the nuclear space which indicates the correct size of implant that should be used. Moving trial spacers in and out of the nuclear cavity numerous times, however, creates further risk of enlarging the incision or damaging the annulus, vertebral endplates and/or even other tissue around the spine whether or not the annulus is present. Also with this procedure, the surgeon wastes time by choosing and obtaining a different trial spacer multiple times and then inserting each trial spacer into the nuclear space. Thus, a need exists for a sizing tool that need not be inserted into a nuclear space multiple times in order to obtain the dimensions of the nuclear space.
Other improvements specifically for the DNP procedure would be desirable. As mentioned above, a DNP requires less extensive surgery than for a TDP since it replaces only part of the disc. Implantation of most known DNPs with pre-formed dimensions generally requires a 5-6 mm, or larger, incision in the annulus for implantation. The incision, however, should be kept as small as possible to hold the DNP within the annulus without using anchors on the DNP that extend into the endplates of the vertebrae for securing the DNP. The minimal invasiveness of the procedure results in minimal recovery and post-surgical pain, and interbody fusion remains a viable revision surgery. Thus, maintaining a small incision and keeping damage to the annulus to a minimum is a high priority. Therefore, it would be desirable to provide a DNP and trial spacer that does not require an enlarged incision and does not significantly damage the annulus or other tissue during insertion and placement of the DNP.
Other problems relate to the geometry of the intervertebral nuclear space. A natural nuclear space within the annulus has a length in the lateral direction (orthogonal to the anterior-posterior direction) that is longer than the width of the space in the anterior-posterior direction. Since it would be desirable to have the sizing tool as well as the implant match the shape of the nuclear space, some conventional sizing tools or implants are generally rectangular, oval or obround with a length greater than its width in order to more closely fit the nuclear space. In this case, the short or narrow side of the sizing tool or implant is presented as the leading edge for insertion (i.e. it faces the incision) in order to maintain a reduced incision on the annulus. This frequently requires an anterior-lateral approach to the surgical site which requires a general surgeon's service, typically in conjunction with an orthopedic surgeon or neurosurgeon, or both, which then raises the costs of the procedure.
A posterior or posterior-lateral approach, while less costly, does not typically permit the access required for inserting sizing tools, removal of the natural disc and implantation of the prosthetic device because the geometry and structure of the spine blocks or fills the path to the nuclear space that is needed for the approach. This is especially true for a sizing tool as described where the sizing tools long side would need to be presented first for insertion and extraction from a posterior approach. Therefore, a need exists for a sizing tool that is not limited to particular surgical approaches.
Some implants utilize an inflatable bladder or balloon-like structure as disclosed by U.S. Patent Publication No. 2004/0133280. These inflated structures, however, are not configured to be deflated in any controlled manner. Uncontrolled collapse of an inflated body within an annulus may result in a deflated structure that is too large or irregularly shaped to be retracted through the incision without enlarging the incision or damaging the annulus or other tissue.
Another problem occurs when the top or bottom end of a sizing tool does not match the geometry of the endplates on the opposing vertebrae which may be slanted to align with the lordotic or kyphotic curve of the spine. When a mismatch occurs, such as when the top plate of a sizing tool remains horizontal while the endplate of a vertebrae it faces is slanted, measurement readings of the height of the nuclear space may be inaccurate.